protobuf: Update protobuf to @laszlocsomor's master at commit 421d909.

Update protobuf to @laszlocsomor's fork of protobuf. This is essentially
the development version of protobuf 3.2.0 and windows specific fixes
from laszlo. Those fixes will be merged into protobuf soon:
protocolbuffers/protobuf#2969).

For instructions on how to update protobuf for bazel see
README.bazel.md.

Change-Id: I4f663955b0baeea3232b69ce32d9292427f5274a

WORKSPACE

@@ -27,6 +27,12 @@ bind(
     actual = "//third_party:guava",
 )
 
+# Used by //third_party/protobuf:protobuf_python
+bind(
+    name = "six",
+    actual = "//third_party/py/six",
+)
+
 # For tools/cpp/test/...
 load("//tools/cpp/test:docker_repository.bzl", "docker_repository")
 docker_repository()
@@ -50,16 +56,15 @@ docker_repository()
 # This allows rules written in skylark to locate apple build tools.
 bind(name = "xcrunwrapper", actual = "@bazel_tools//tools/objc:xcrunwrapper")
 
-new_local_repository(
+local_repository(
     name = "com_google_protobuf",
-    path = "./third_party/protobuf/3.0.0/",
-    build_file = "./third_party/protobuf/3.0.0/BUILD",
+    path = "./third_party/protobuf/",
 )
 
 new_local_repository(
     name = "com_google_protobuf_java",
-    path = "./third_party/protobuf/3.0.0/",
-    build_file = "./third_party/protobuf/3.0.0/com_google_protobuf_java.BUILD",
+    path = "./third_party/protobuf/",
+    build_file = "./third_party/protobuf/com_google_protobuf_java.BUILD",
 )
 
 # OpenJDK distributions used to create a version of Bazel bundled with the OpenJDK.

src/main/protobuf/BUILD

@@ -1,7 +1,7 @@
 package(default_visibility = ["//visibility:public"])
 
 load("//tools/build_rules:genproto.bzl", "java_proto_library", "cc_grpc_library")
-load("//third_party/protobuf/3.0.0:protobuf.bzl", "cc_proto_library", "py_proto_library")
+load("//third_party/protobuf:protobuf.bzl", "cc_proto_library", "py_proto_library")
 
 FILES = [
     "build",